The lithium secondary battery has recently been widely used, for example, as an electric source for driving small-sized electronic devices. The lithium secondary battery comprises a positive electrode, a negative electrode and a non-aqueous electrolytic solution. The positive electrode generally comprises a complex oxide of lithium such as, LiCoO2, and the negative electrode generally comprises a carbonaceous material or metallic lithium. A carbonate such as ethylene carbonate (EC) and propylene carbonate (PC) has favorably been used in the non-aqueous electrolytic solution for the lithium secondary battery.
The lithium secondary battery requires a further improvement on battery performances such as cycle characteristics of the battery and electric capacity as well as safety.
Japanese Patent Provisional Publication No. H10-275632 discloses that a lithium secondary battery improved in safety can be provided by containing an aromatic ester such as trimellitic ester or phthalic ester, or an alkylbenzene compound such as toluene or butylbenzene (including tert-butylbenzene). However, a further improvement is required on cycle performance. Particularly, the cycle performance and storage stability at high temperatures are insufficient in the case that the lithium secondary battery is used at a high voltage charge potential of 4.2 V or more or at a higher energy density. Further, such a problem has been found that a gas is generated to expand the battery under the above-mentioned conditions.
Japanese Patent Provisional Publication No. 2002-298909 and the pamphlet of WO 02/29922 describe that cycle characteristics, electric capacity and storage characteristics of the lithium secondary battery can be improved by adding a compound having a tert-alkyl group having 5 or more carbon atoms such as tert-pentylbenzene as the tert-alkylbenzene compound to an electrolyte solvent of a lithium secondary battery.
A process for preparation of tert-alkylbenzene compound has been known. For example, tert-pentylbenzene has been prepared by (1) a process of reacting benzene with isoamyl halide in the presence of an acid catalyst (disclosed in J. Am. Chem. Soc., 74, 292(1952)), (2) a process of reacting benzene with isoamylene in the presence of an acid catalyst (disclosed in a. Am. Chem. Soc., 78, 2000(1956)), or (3) a process of reacting cumene with ethylene in the presence of an alkali catalyst (disclosed in U.S. Pat. No. 4,179,472).
The present inventors have studied and found that the tert-alkylbenzene compound prepared according to the known processes contains a trace amount of by-products as impurities. The by-product is a benzene compound having a benzene ring substituted with a hydrocarbon group having 1 to 4 carbon atoms via at least one tertiary carbon atom. Meanwhile, it is difficult to obtain an alkyl halide of high purity or an isoalkylene of high purity. The inventors have further found that the impurities are inevitably produced by a side reaction, even if the materials of high purity were reacted. Moreover, the impurities have boiling points near those of the tert-alkylbenzene compounds. If once the produced impurities are mixed with the product, it is industrially difficult to isolate or purify the product. If the reaction product were isolated and purified from the impurities in a conventional manner, the reaction yield would greatly decrease.